Genome-wide sequencing reveals geographical variations in the thermal adaptation of an aquaculture species with frequent seedling introductions.

Climate change and human activity are essential factors affecting marine biodiversity and aquaculture, and understanding the impacts of human activities on the genetic structure to increasing high temperatures is crucial for sustainable aquaculture and marine biodiversity conservation. As a commercially important bivalve, the Manila clam Ruditapes philippinarum is widely distributed along the coast of China, and it has been frequently introduced from Fujian Province, China, to other regions for aquaculture. In this study, we collected four populations of Manila clams from different areas to evaluate their thermal tolerance by measuring cardiac performance and genetic variations using whole-genome resequencing. The upper thermal limits of the clams showed high variations within and among populations. Different populations displayed divergent genetic compositions, and the admixed population was partly derived from the Zhangzhou population in Fujian Province, implying a complex genomic landscape under the influence of local genetic sources and human introductions. Multiple single nucleotide polymorphisms (SNPs) were associated with the cardiac functional traits, and some of these SNPs can affect the codon usage and the structural stability of the resulting protein. This study shed light on the importance of establishing long-term ecological and genetic monitoring programs at the local level to enhance resilience to future climate change.

Characterization and Functional Analysis of Fads Reveals Δ5 Desaturation Activity during Long-Chain Polyunsaturated Fatty Acid Biosynthesis in Dwarf Surf Clam Mulinia lateralis.

Fatty acid desaturases (Fads), as key enzymes in the biosynthesis of long-chain polyunsaturated fatty acids (LC-PUFAs), catalyze the desaturation between defined carbons of fatty acyl chains and control the degree of unsaturation of fatty acids. In the present study, two Fads genes, designated MulFadsA and MulFadsB, were identified from the genome of the dwarf surf clam Mulinia lateralis (Mollusca, Mactridae), and their spatiotemporal expression was examined. MulFadsA and MulFadsB contained the corresponding conserved functional domains and clustered closely with their respective orthologs from other mollusks. Both genes were expressed in the developmental stages and all tested adult tissues of M. lateralis, with MulFadsA exhibiting significantly higher expression levels in adult tissues than MulFadsB. Subsequently, the effects of dietary microalgae on Fads expressions in the dwarf surf clam were investigated by feeding clams with two types of unialgal diets varying in fatty acid content, i.e., Chlorella pyrenoidosa (Cp) and Platymonas helgolandica (Ph). The results show that the expressions of MulFads were significantly upregulated among adult tissues in the Cp group compared with those in the Ph group. In addition, we observed the desaturation activity of MulFadsA via heterologous expression in yeasts, revealing Δ5 desaturation activity toward PUFA substrates. Taken together, these results provide a novel perspective on M. lateralis LC-PUFA biosynthesis, expanding our understanding of fatty acid synthesis in marine mollusks.

The First High-Quality Genome Assembly of Freshwater Pearl Mussel Sinohyriopsis cumingii: New Insights into Pearl Biomineralization.

China leads the world in freshwater pearl production, an industry in which the triangle sail mussel (Sinohyriopsis cumingii) plays a pivotal role. In this paper, we report a high-quality chromosome-level genome assembly of S. cumingii with a size of 2.90 Gb-the largest yet reported among bivalves-and 89.92% anchorage onto 19 linkage groups. The assembled genome has 37,696 protein-coding genes and 50.86% repeat elements. A comparative genomic analysis revealed expansions of 752 gene families, mostly associated with biomineralization, and 237 genes under strong positive selection. Notably, the fibrillin gene family exhibited gene family expansion and positive selection simultaneously, and it also exhibited multiple high expressions after mantle implantation by transcriptome analysis. Furthermore, RNA silencing and an in vitro calcium carbonate crystallization assay highlighted the pivotal role played by one fibrillin gene in calcium carbonate deposition and aragonite transformation. This study provides a valuable genomic resource and offers new insights into the mechanism of pearl biomineralization.

Identification and involvement of DAX1 gene in spermatogenesis of boring giant clam Tridacna crocea.

DAX1 (dosage-sensitive sex reversal, adrenal hypoplasia congenital critical region on X chromosome gene 1), a key sex determinant in various species, plays a vital role in gonad differentiation and development and controls spermatogenesis. However, the identity and function of DAX1 are still unclear in bivalves. In the present study, we identified a DAX1 (designed as Tc-DAX1) gene from the boring giant clam Tridacna crocea, a tropical marine bivalve. The full length of Tc-DAX1 was 1877 bp, encoding 462 amino acids, with a Molecular weight of 51.81 kDa and a theoretical Isoelectric point of 5.87 (pI). Multiple sequence alignments and phylogenetic analysis indicated a putative ligand binding domain (LBD) conserved regions clustered with molluscans DAX1 homologs. The tissue distributions in different reproductive stages revealed a dimorphic pattern, with the highest expression trend in the male reproductive stage, indicating its role in spermatogenesis. The DAX1 expression data from embryonic stages shows its highest expression profile (P < 0.05) in the zygote stage, followed by decreasing trends in the larvae stages (P > 0.05). The localization of DAX1 transcripts has also been confirmed by whole mount in situ hybridization, showing high positive signals in the fertilized egg, 2, and 4-cell stage, and gastrula. Moreover, RNAi knockdown of the Tc-DAX1 transcripts shows a significantly lower expression profile in the ds-DAX1 group compared to the ds-EGFP group. Subsequent histological analysis of gonads revealed that spermatogenesis was affected in a ds-DAX1 group compared to the ds-EGFP group. All these results indicate that Tc-DAX1 is involved in the spermatogenesis and early embryonic development of T. crocea, providing valuable information for the breeding and aquaculture of giant clams.

Comprehensive Multi-omics Approaches Provide Insights to Summer Mortality in the Clam Meretrix petechialis.

Bivalve mass mortalities have been reported worldwide, which not only can be explained as a result of pathogen infection, but may reflect changes in environments. Although these episodes were often reported, there was limited information concerning the molecular responses to various stressors leading to summer mortality. In the present work, RNA sequencing (RNA-seq), tandem mass tagging (TMT)-based quantitative proteomics, and 16S rRNA sequencing were used to explore the natural outbreak of summer mortality in the clam Meretrix petechialis. We identified a total of 172 differentially expressed genes (DEGs) and 222 differentially expressed proteins (DEPs) in the diseased group compared to the normal group. The inconsistent expression profiles of immune DEGs/DEPs may be due to the immune dysregulation of the diseased clams. Notably, 11 solute carrier family genes were found among the top 20 down-regulated genes in the diseased group, indicating that weakened transmembrane transport ability might occur in the diseased clams. Integration analysis of transcriptomic and proteomic results showed that many metabolic processes such as "arginine and proline metabolism" and "tyrosine metabolism" were inhibited in the diseased group, suggesting metabolic inhibition. Moreover, 16S rRNA sequencing revealed that the microbial composition of clam hepatopancreas was disordered in the diseased group. The comparison of DEGs expression between the natural summer mortality event and an artificial challenge experiment involving both Vibrio infection and heat stress revealed 9/15 genes showing similar expression trends between the two conditions, suggesting that the summer mortality might be caused by a combination of high temperature and Vibrio infection. These results would deepen our understanding of summer mortality and provide candidate resistance markers for clam resistance breeding.

Comprehensive investigation of differentially expressed ncRNAs, mRNAs, and their ceRNA networks in the regulation of shell color formation in clam, Cyclina sinensis.

Noncoding RNAs (ncRNAs) have gained significant attention in recent years due to their crucial roles in various biological processes. However, our understanding of the expression and functions of ncRNAs in Cyclina sinensis, an economically important marine bivalve, remains limited. This study aimed to address this knowledge gap by systematically identifying ncRNAs in the mantles of C. sinensis with purple and white shells. Through our analysis, we identified a differential expression of 1244 mRNAs, 196 lncRNAs, 49 circRNAs, and 23 miRNAs between purple- and white-shell clams. Functional enrichment analysis revealed the involvement of these differentially expressed ncRNAs in biomineralization and pigmentation processes. To gain further insights into the regulatory mechanisms underlying shell color formation, we established competitive endogenous RNA (ceRNA) networks. These networks allowed us to identify targeted differentially expressed miRNAs (DEMis) and genes associated with shell color formation. Based on the ceRNA networks, we obtained an up-down-up lncRNA-miRNA-mRNA network consisting of 13 upregulated lncRNAs and a circRNA-miRNA-mRNA network comprising three upregulated circRNAs (novel_circ_0004787, novel_circ_0001165, novel_circ_0000245). Through these networks, we identified and selected an upregulated novel gene (evm.TU.Hic_asm_7.988) and a downregulated sponge miRNA (hru-miR-1985) as potential contributors to shell color regulation. In summary, the present investigation offers a comprehensive analysis of ncRNA catalogs expressed in the clam mantle of C. sinensis. The findings enhance our comprehension of the molecular mechanisms governing shell coloration and offer new perspectives for selective breeding of C. sinensis in the future.

Trans-2-nonadecyl-4-(hydroxymethyl)-1,3-dioxolane (TNHD) purified from freshwater clams markedly alleviates dimethylnitrosamine-induced hepatic fibrosis.

Liver fibrosis occurs due to injury or inflammation, which results in the excessive production of collagen and the formation of fibrotic scar tissue that impairs liver function. Despite the limited treatment options available, freshwater clams may hold promise in the treatment of liver fibrosis. In this study, we demonstrated the effects of ethanol extract of freshwater clam (FCE), ethyl acetate extract of FCE (EA-FCE), and trans-2-nonadecyl-4-(hydroxymethyl)-1,3-dioxolane (TNHD) on liver fibrosis induced by dimethylnitrosamine (DMN). Administration of FCE and TNHD alleviated liver injury, including tissue damage, necrosis, inflammation scores, fibrosis scores, serum enzymes, and triglyceride levels. Furthermore, we analyzed the expression of fibrosis-related proteins, such as α-smooth muscle actin (α-SMA) and transforming growth factor (TGF-ß), as well as the hydroxyproline content, which decreased after treatment with FCE and TNHD. Animal experiments revealed that FCE and TNHD can reduce liver fibrosis by inhibiting cytokines that activate stellate cells and decreasing extracellular matrix (ECM) secretion. Cell experiments have shown that TNHD inhibits the MAPK/Smad signaling pathway and TGF-ß1 activation, resulting in a reduction in the expression of fibrosis-related proteins. Therefore, freshwater clam extracts, particularly TNHD, may have potential therapeutic and preventive effects for the amelioration of liver fibrosis.

An historical "wreck": A transcriptome assembly of the naval shipworm, Teredo navalis Linnaeus, 1978.

Historically famous for their negative impact on human-built marine wood structures, mollusc shipworms play a central ecological role in marine ecosystems. Their association with bacterial symbionts, providing cellulolytic and nitrogen-fixing activities, underscores their exceptional wood-eating and wood-boring behaviours, improving energy transfer and the recycling of essential nutrients locked in the wood cellulose. Importantly, from a molecular standpoint, a minute of omic resources are available from this lineage of Bivalvia. Here, we produced and assembled a transcriptome from the globally distributed naval shipworm, Teredo navalis (family Teredinidae). The transcriptome was obtained by sequencing the total RNA from five equidistant segments of the whole body of a T. navalis specimen. The quality of the produced assembly was accessed with several statistics, revealing a highly contiguous (1194 N50) and complete (over 90% BUSCO scores for Eukaryote and Metazoan databases) transcriptome, with nearly 38,000 predicted ORF, more than half being functionally annotated. Our findings pave the way to investigate the unique evolutionary biology of these highly modified bivalves and lay the foundation for an adequate gene annotation of a full genome sequence of the species.

Transcriptome analysis provides new insights into the immune response of Ruditapes philippinarum infected with Vibrio alginolyticus.

Manila clam (Ruditapes philippinarum) is a bivalve species with commercial value, but it is easily infected by pathogenic microorganisms in aquaculture, which restricts the shellfish industry. Notably, the impact of Vibrio alginolyticus on clam culture is obvious. In this study, RNA-seq was performed to analyze clam hepatopancreas tissue in 48 h (challenge group, G48h) and 96 h (challenge group, G96h) after infection with V. alginolyticus and 0 h after injection of PBS (control group, C). The results showed that a total of 1670 differentially expressed genes were detected in the G48h vs C group, and 1427 differentially expressed genes were detected in the G96h vs C group. In addition, KEGG analysis showed that DEGs were significantly enriched in pathways such as Lysosome and Mitophagy. Moreover, 15 immune related DEGs were selected for qRT-PCR analysis to verify the accuracy of RNA-seq, and the results showed that the expression level of DEGs was consistent with that of RNA-seq. Therefore, the results obtained in this study provides a preliminary understanding of the immune defense of R. philippinarum and molecular insights for genetic breeding of V. alginolyticus resistance in Manila clam.

Chromosome-level genome assembly and annotation of rare and endangered tropical bivalve, Tridacna crocea.

Tridacna crocea is an ecologically important marine bivalve inhabiting tropical coral reef waters. High quality and available genomic resources will help us understand the population structure and genetic diversity of giant clams. This study reports a high-quality chromosome-scale T. crocea genome sequence of 1.30 Gb, with a scaffold N50 and contig N50 of 56.38 Mb and 1.29 Mb, respectively, which was assembled by combining PacBio long reads and Hi-C sequencing data. Repetitive sequences cover 71.60% of the total length, and a total of 25,440 protein-coding genes were annotated. A total of 1,963 non-coding RNA (ncRNA) were determined in the T. crocea genome, including 62 micro RNA (miRNA), 58 small nuclear RNA (snRNA), 83 ribosomal RNA (rRNA), and 1,760 transfer RNA (tRNA). Phylogenetic analysis revealed that giant clams diverged from oyster about 505.7 Mya during the evolution of bivalves. The genome assembly presented here provides valuable genomic resources to enhance our understanding of the genetic diversity and population structure of giant clams.

Sensing and regulation of long-chain polyunsaturated fatty acids pool in marine mollusks: Characterization of UBXD8 from the razor clam Sinonovacula constricta.

The razor clam Sinonovacula constricta is known for its richness in long-chain polyunsaturated fatty acids (LC-PUFA, C ≥ 20). Previously, we demonstrated that it possesses a complete LC-PUFA biosynthetic pathway. However, the mechanisms by which it senses the LC-PUFA pool to regulate their biosynthesis remain unclear. Here, we presented the LC-PUFA sensor UBXD8 as a critical molecule in this intriguing process. The S. constricta UBXD8 (ScUBXD8) shared all characteristic features of its mammalian counterpart and exhibited high mRNA levels in digestive tissues, suggesting its functional role in this bivalve species. By purification of ScUBXD8 protein in vitro, we discovered its ability to sense unsaturated fatty acids (UFA, C ≥ 14) but not saturated ones, as evidenced by polymerization detection. Furthermore, the intensity of ScUBXD8 polymerization increased progressively with longer acyl chain lengths, greater unsaturation degrees, and higher UFA concentrations. Exceptionally, for those located at the same node in LC-PUFA biosynthetic pathway, ScUBXD8 displayed a stronger sensitivity to n-6 UFA compared to n-3 UFA. These results suggested a critical role for ScUBXD8 in balancing fatty acids composition and ratio of n-6/n-3 UFA in S. constricta. Moreover, the UAS domain was confirmed essential for ScUBXD8 polymerization. Through knockdown of ScUbxd8 gene in vivo, there were significant shifts in expression patterns of genes related to LC-PUFA biosynthesis, concurrently influencing fatty acids compositions. These results suggested that ScUBXD8 likely plays a regulatory role in LC-PUFA biosynthesis, possibly through the INSIG-SREBP pathway. Collectively, this study proposed that S. constricta might maintain LC-PUFA homeostasis through UBXD8 to regulate their biosynthesis.

Development and validation of genome-wide SSR molecular markers of Tapes dorsatus.

BACKGROUND: Tapes dorsatus is an economically important benthic animal in the Beibu Gulf of China. However, the deficiency of microsatellite markers has hindered the study of its genetics. The development of microsatellite markers will provide useful tools for genetic improvement, variety identification, phylogenetic analysis and resource conservation. METHODS AND RESULTS: Within the genome sequence, 145,008 simple sequence repeats (SSRs) were identified, and 29,691 primer pairs were designed successfully. A total of 100 primer pairs were randomly synthesized for testing, and 93 primers yielded products. Sixty highly polymorphic primers were used to reveal the genetic diversity of 50 T. dorsatus individuals. The average number of alleles (Na) of the population was 10.40; the average number of effective alleles was 6.16, the average expected heterozygosity (He) was 0.82, and the average polymorphic information content was 0.80. The genetic structure of the population was detected, by which the population could be divided into three subpopulations. CONCLUSION: We identified 145,008 SSRs in the genome of T. dorsatus and designed 29,691 primer pairs in this study. Of 100 synthesized primers, 60 were highly polymorphic and used to reveal the genetic diversity and structure of the population. The SSR markers identified here will provide useful tools and a foundation for genetic diversity, linkage mapping and molecular marker-aided breeding in T. dorsatus.

Hiding from heat: The transcriptomic response of two clam species is modulated by behaviour and habitat.

Rising occurrence of extreme warming events are profoundly impacting ecosystems, altering their functioning and services with significant socio-economic consequences. Particularly susceptible to heatwaves are intertidal shellfish beds, located in estuarine areas already stressed by factors such as rainfall events, red tides, eutrophication, and pollution. In Galicia, Northwestern Spain, these beds support vital shellfisheries, featuring the native clam Ruditapes decussatus and the non-indigenous R. philippinarum. Over recent decades, these populations have experienced notable abundance shifts due to various anthropogenic impacts, including climate change. In this habitat, patches of the seagrass Zostera noltei that coexist with bare sand can act as thermal refuges for benthic organisms such as clams. To assess the impact of heatwaves on these ecosystems, a mesocosm experiment was conducted. Juveniles of both clam species in two habitat types-bare sand and sand with Z. noltei-were exposed to simulated atmospheric heatwaves during diurnal low tide for four consecutive days. Subsequent transcriptomic analysis revealed that high temperatures had a more pronounced impact on the transcriptome of R. philippinarum compared to R. decussatus. The habitat type played a crucial role in mitigating heat stress in R. philippinarum, with the presence of Z. noltei notably ameliorating the transcriptomic response. These findings have direct applications in shellfishery management, emphasizing the importance of preserving undisturbed patches of Z. noltei as thermal refuges, contributing to the mitigation of heatwave effects on shellfish populations.

Eyeless razor clam Sinonovacula constricta discriminates light spectra through opsins to guide Ca2+ and cAMP signaling pathways.

Phototransduction is based on opsins that drive distinct types of Gα cascades. Although nonvisual photosensitivity has long been known in marine bivalves, the underlying molecular basis and phototransduction mechanism are poorly understood. Here, we introduced the eyeless razor clam Sinonovacula constricta as a model to clarify this issue. First, we showed that S. constricta was highly diverse in opsin family members, with a significant expansion in xenopsins. Second, the expression of putative S. constricta opsins was highly temporal-spatio specific, indicating their potential roles in S. constricta development and its peripheral photosensitivity. Third, by cloning four S. constricta opsins with relatively higher expression (Sc_opsin1, 5, 7, and 12), we found that they exhibited different expression levels in response to different light environments. Moreover, we demonstrated that these opsins (excluding Sc_opsin7) couple with Gαq and Gαi cascades to mediate the light-dependent Ca2+ (Sc_opsin1 and 5) and cAMP (Sc_opsin12) signaling pathways. The results indicated that Sc_opsin1 and 5 belonged to Gq-opsins, Sc_opsin12 belonged to Gi-opsins, while Sc_opsin7 might act as a photo-isomerase. Furthermore, we found that the phototransduction function of S. constricta Gq-opsins was dependent on the lysine at the seventh transmembrane domain, and greatly influenced by the external light spectra in a complementary way. Thus, a synergistic photosensitive system mediated by opsins might exist in S. constricta to rapidly respond to the transient or subtle changes of the external light environment. Collectively, our findings provide valuable insights into the evolution of opsins in marine bivalves and their potential functions in nonvisual photosensitivity.

The transcriptomic and biochemical responses of blood clams (Tegillarca granosa) to prolonged intermittent hypoxia.

The blood clam (Tegillarca granosa), a marine bivalve of ecological and economic significance, often encounters intermittent hypoxia in mudflats and aquatic environments. To study the response of blood clam foot to prolonged intermittent hypoxia, the clams were exposed to intermittent hypoxia conditions (0.5 mg/L dissolved oxygen, with a 12-h interval) for 31 days. Initially, transcriptomic analysis was performed, uncovering a total of 698 differentially expressed genes (DEGs), with 236 upregulated and 462 downregulated. These genes show enrichments in signaling pathways related to glucose metabolism, sugar synthesis and responses to oxidative stress. Furthermore, the activity of the enzyme glutathione peroxidase (GPx) and the levels of gpx1 mRNA showed gradual increases, reaching their peak on the 13th day of intermittent hypoxia exposure. This observation suggests an indirect protective role of GPx against oxidative stress. The results of this study make a significantly contribute to our broader comprehensive of the physiological, biochemical responses, and molecular reactions governing the organization of foot muscle tissue in marine bivalves exposed to prolonged intermittent hypoxic conditions.

Glacial vicariance and secondary contact shape demographic histories in a freshwater mussel species complex.

Characterizing the mechanisms influencing the distribution of genetic variation in aquatic species can be difficult due to the dynamic nature of hydrological landscapes. In North America's Central Highlands, a complex history of glacial dynamics, long-term isolation, and secondary contact have shaped genetic variation in aquatic species. Although the effects of glacial history have been demonstrated in many taxa, responses are often lineage- or species-specific and driven by organismal ecology. In this study, we reconstruct the evolutionary history of a freshwater mussel species complex using a suite of mitochondrial and nuclear loci to resolve taxonomic and demographic uncertainties. Our findings do not support Pleurobema rubrum as a valid species, which is proposed for listing as threatened under the U.S. Endangered Species Act. We synonymize P. rubrum under Pleurobema sintoxia-a common and widespread species found throughout the Mississippi River Basin. Further investigation of patterns of genetic variation in P. sintoxia identified a complex demographic history, including ancestral vicariance and secondary contact, within the Eastern Highlands. We hypothesize these patterns were shaped by ancestral vicariance driven by the formation of Lake Green and subsequent secondary contact after the last glacial maximum. Our inference aligns with demographic histories observed in other aquatic taxa in the region and mirrors patterns of genetic variation of a freshwater fish species (Erimystax dissimilis) confirmed to serve as a parasitic larval host for P. sintoxia. Our findings directly link species ecology to observed patterns of genetic variation and may have significant implications for future conservation and recovery actions of freshwater mussels.

Comprehensive analysis of differentially expressed mRNA, lncRNA and miRNA, and their ceRNA networks in the regulation of shell color in the Manila clam (Ruditapes philippinarum).

The regulatory mechanism of ceRNA network plays an important role in molecular function and biological processes, however, the molecular mechanism in the shell color of Ruditapes philippinarum has not yet been reported. In this study, we performed transcriptome sequencing on the mantle of R. philippinarum with different shell colors, and screened for mRNA, miRNA, and lncRNA. A total of 61 mRNAs, 3725 lncRNAs and 90 miRNAs were obtained from all the shell color comparison groups (all mRNAs, lncRNAs and miRNAs P < 0.05), and 7 mRNAs, 8 lncRNAs, and 4 miRNAs of the porphyrin pathway and melanin pathway were screened for competitive endogenous RNA (ceRNA) network construction. The results indicate that the ceRNA network composed of mRNA and lncRNA, centered around efu-miR-101, mle-bantam-3p, egr-miR-9-5p, and sma-miR-75p, may play a crucial regulatory role in shell color formation. This study reveals for the first time the mechanism of ceRNA regulatory networks in the shell color of R. philippinarum and providing important reference data for molecular breeding of shell color in R. philippinarum.

eDNA-based detection reveals invasion risks of a biofouling bivalve in the world's largest water diversion project.

Environmental DNA (eDNA) has increasingly been used to detect rare species (e.g., newly introduced nonindigenous species) in both terrestrial and aquatic ecosystems, often with distinct advantages over traditional methods. However, whether water eDNA signals can be used to inform invasion risks remains debatable owing to inherent uncertainties associated with the methods used and the varying conditions among study systems. Here, we sampled eDNA from canals of the central route of the South-to-North Water Diversion Project (hereafter SNWDP) in China to investigate eDNA distribution and efficacy to inform invasion risks in a unique lotic system. We first conducted a total of 16 monthly surveys in this system (two sites in the source reservoir and four sites in the main canal) to test if eDNA could be applied to detect an invasive, biofouling bivalve, the golden mussel Limnoperna fortunei. Second, we initiated a one-time survey in a sub-canal of the SNWDP using refined sampling (12 sites in ~22 km canal) and considered a few environmental predictors. We found that detection of target eDNA in the main canal was achieved up to 1100 km from the putative source population but was restricted to the warmer months (May-November). Detection probability exhibited a significant positive relationship with average daily minimum air temperature and with water temperature, consistent with the expected spawning season. eDNA concentration in the main canal generally fluctuated across months and sites and was generally higher in warmer months. Golden mussel eDNA concentration in the sub-canal decreased significantly with distance from the source and with increasing water temperature and became almost undetectable at ~22 km distance. Given the enormity of the SNWDP, golden mussels may eventually expand their distribution in the main canal, with established "bridgehead" populations facilitating further spread. Our findings suggest an elevated invasion risk of golden mussels in the SNWDP in warm months, highlighting the critical period for spread and, possibly, management.

Microbiome profile of the Antarctic clam Laternula elliptica.

The filter feeder clam Laternula elliptica is a key species in the Antarctic ecosystem. As a stenothermal benthic species, it has a poor capacity for adaptation to small temperature variations. Despite their ecological importance and sensitivity to climate change, studies on their microbiomes are lacking. The goal of this study was to characterize the bacterial communities of L. elliptica and the tissues variability of this microbiome to provide an initial insight of host-microbiota interactions. We investigated the diversity and taxonomic composition of bacterial communities of L. elliptica from five regions of the body using high-throughput 16S rRNA gene sequencing. The results showed that the microbiome of L. elliptica tended to differ from that of the surrounding seawater samples. However, there were no significant differences in the microbial composition between the body sites, and only two OTUs were present in all samples, being considered core microbiome (genus Moritella and Polaribacter). No significant differences were detected in diversity indexes among tissues (mean 626.85 for observed OTUs, 628.89 Chao1, 5.42 Shannon, and 0.87 Simpson). Rarefaction analysis revealed that most tissues reached a plateau of OTU number according to sample increase, with the exception of Siphon samples. Psychromonas and Psychrilyobacter were particularly abundant in L. elliptica whereas Fluviicola dominated seawater and siphons. Typical polar bacteria were Polaribacter, Shewanella, Colwellia, and Moritella. We detected the prevalence of pathogenic bacterial sequences, particularly in the family Arcobacteraceae, Pseudomonadaceae, and Mycoplasmataceae. The prokaryotic diversity was similar among tissues, as well as their taxonomic composition, suggesting a homogeneity of the microbiome along L. elliptica body. The Antarctic clam population can be used to monitor the impact of human activity in areas near Antarctic stations that discharge wastewater.

Effects of acute hypoxia and reoxygenation on histological structure, antioxidant response, and apoptosis in razor clam Sinonovacula constricta.

Hypoxia is one of the major environmental problems limiting the healthy development of intensive aquaculture. Marine benthic shellfish are encountering heightened problems related to hypoxic stress as a result of ongoing human activities and aquaculture operations. Razor clam Sinonovacula constricta, a commercially valuable shellfish, has not yet been reported in studies on physiological changes caused by hypoxia and reoxygenation. To understand the negative effects of hypoxia and reoxygenation on the clams, we set up two low-oxygen concentration groups (DO 2.0 mg/L and DO 0.5 mg/L) and assessed multiple aspects of oxidative damage to their hepatopancreas and gills. After the hypoxic stress, the two tissues of the razor clam suffered varying degrees of damage, including cell degeneration and disruption of mitochondrial cristae. After reoxygenation, the 2.0 mg/L group recovered substantially, but the clams in the 0.5 mg/L group still unrecovered. The activities of antioxidant enzymes (MDA, T-AOC, SOD, GPX, and CAT) in clams were considerably altered by acute hypoxia and reoxygenation. Briefly, there was a growing and then declining trend in MDA, T-AOC, and SOD activities in the hepatopancreas, whereas GPX and CAT activities showed the converse trend. In the hepatopancreas and gills, the level of anti-apoptotic gene Bcl-2 transcripts gradually decreased with the duration of hypoxia and increased following reoxygenation. However, changes in the transcript level of the pro-apoptotic gene Bax were in contrast to that of Bcl-2. The TUNEL assay revealed that hypoxia caused apoptosis. Furthermore, at DO 0.5 mg/L, the degree of apoptosis was more significant than at DO 2.0 mg/L, and hepatopancreatic apoptosis was more severe than gill apoptosis. Collectively, our findings imply that hypoxia induces oxidative stress, histological damage, and apoptosis in razor clams in a concentration-dependent and tissue-specific manner. These consequences serve as a reminder that prolonged recovery periods may be required for razor clams to fully recover from oxidative damage resulting from hypoxia-reoxygenation episodes.